1. Field of the Invention
This invention relates to thermal stabilization of a single cavity structure, or a multiple cavity structure (wherein cylindrical cavities are arranged coaxially in tandem, as in the construction of a microwave filter of plural resonant chambers, or cavities), and, more particularly, to an arrangement of one or more cavities employing at least one traverse bowed end well including materials with differing coefficients of thermal expansion to provide selected ratios of thermally induced deformation of the end wall to counteract changes in resonance induced by thermal expansion/contraction of an outer cylindrical wall of the cavity structure.
2. Description of Related Art
Cavity structures are employed for microwave filters. As is known in the art, a cavity resonator is, in effect, a tuned circuit which is utilized to filter electromagnetic signals of unwanted frequencies from input electromagnetic energy and to output signals having a preselected bandwidth centered about one or more resonant frequencies. A cavity which is frequently employed for a cavity resonator has the shape of a right circular cylinder wherein the diameter and the height (or the axial length) of the cavity together determine the value of a resonant frequency. For filters described mathematically as multiple pole filters, it is common practice to provide a cylindrical housing with transverse disc shaped partitions or walls defining the individual cavities. Irises in the partitions provide for coupling of desired modes of electromagnetic waves between the cavities to provide a desired filter function or response.
A problem arises in that changes in environmental temperature induce changes in the dimensions of the filter with a consequent shift in the resonant frequency of each filter section. Because the resonant frequency associated with each cavity is a function of the cavity's dimensions, an increase in temperature will cause dimensional changes in the cavity and, therefore, temperature-induced changes in the resonant frequency associated with the cavity. Specifically, an increasing temperature will cause thermal expansion of the waveguide body to enlarge the cavity both axially and transversely.
A filter fabricated of aluminum undergoes substantial dimensional changes as compared to a filter constructed of invar nickel-steel alloy (herein referred to as "INVAR") due to the much larger thermal coefficient of expansion for aluminum as compared to INVAR. However, it is often the case that aluminum is nevertheless a preferable material for constructing filters, especially for aerospace applications, due to its lower density, as well as its greater ability to dissipate heat, as compared to that of INVAR.
A solution to the foregoing problem, useful especially for a two-cavity filter, is presented in U.S. Pat. No. 4,677,403 of Kich (hereinafter, "the '403 patent"), the entirety of which is hereby incorporated by reference. Therein, an end wall of each cavity is formed of a bowed disc, while a central wall having an iris for coupling electromagnetic energy has a planar form. An increase of temperature enlarges the diameter of each cavity, and also increases the bowing of the end walls, with a consequent reduction in the axial length of each cavity. The resonant frequency shift associated with the increased diameter is counterbalanced by the shift associated with the decrease in length. Similar compensation occurs during a reduction in temperature wherein the diameter decreases and the length increases.
Another approach is presented in U.S. Pat. No. 5,374,911 of Kich et al. (hereinafter, "the '911 patent"), the entirety of which is hereby incorporated by reference, and which discloses a cylindrical filter structure of multiple cavities with a succession of transverse walls defining the cavities. Selected ones of the transverse walls provide for thermal compensation. Each of the selected transverse walls is fabricated of a bowed disc encircled by a ring formed of material of lower thermal expansion coefficient than the material of the transverse wall. Inner ones of the transverse walls are provided with irises for coupling electromagnetic power between successive ones of the cavities. By varying the composition of the rings to attain differing coefficients of thermal expansion within the rings, different amounts of bowing occur in the corresponding transverse discs with changes in temperature. Thus, the ring of an inner transverse wall has a relatively large coefficient of thermal expansion as compared to the ring of an outer one of the transverse walls, resulting in a lesser amount of bowing of the inner wall and a larger amount of bowing of the outer wall with increase in environmental temperature and temperature of the filter.
In a preferred embodiment disclosed in the '911 patent, the housing is constructed of aluminum, as is a central planar transverse wall having a coupling iris. The other transverse walls, both to the right and to the left of the central wall, are provided with a bowed structure, the bowed walls being encircled by metallic rings. The inboard rings nearest the central wall are fabricated of titanium, and the outboard rings are fabricated of INVAR. The INVAR has a lower coefficient of thermal expansion than does the titanium and, accordingly, the peripheral portions of the outboard walls, in the case of a four-cavity structure, experience a more pronounced bowing upon a increase in environmental temperature than do the inner walls which are bounded by the titanium rings having a larger coefficient of thermal expansion.
The reason for the use of the rings of differing coefficients of thermal expansion is as follows. Deflection of an inboard wall reduces the axial length of an inner cavity, on the inner side of the wall, while increasing the axial length of an outer cavity, on the opposite side of the wall, with increasing temperature. Thus, the inboard wall acts in the correct sense to stabilize the inner cavity but in the incorrect sense for stabilization of the outer cavity. Accordingly, in stabilizing the outer cavity by means of the outer wall, it is necessary to provide an additional bowing to overcome the movement of the inboard wall, to thereby stabilize thermally the outer cavity.
One disadvantage associated with a resonator structure constructed in accordance with either the '403 patent or the '911 patent is that the relatively thin aluminum disk used for the end wall, that is capable of bowing in response to increased temperature, has a tendency to exhibit undesirable thermal gradients across the surface of the end wall, resulting in a frequency shift when RF power is applied.
Accordingly, there is a need for an electromagnetic resonator end wall assembly configured so as to minimize or eliminate the aforementioned problems.